Solidification microstructure of SiC particles reinforced Zn-Al composites under different kinds of ultrasonic exposure was investigated and the origin of the microstructural changes induced by ultrasonic was explained. Under continuous ultrasonic exposure, highly refined and non-dendritic globular matrix structure was achieved but serious particle clustering occurred at the top and peripheral parts of the solidified ingot. Homogeneous particle distribution as well as dramatic grain refinement of matrix metal was observed in the ingot solidified with isothermal ultrasonic processing. Strong supportive evidence showed that acoustically induced dendrite fragmentation was dominantly responsible for the formation of a refined and globular microstructure. The particle clustering was closely related to the pattern of the acoustic streaming and the interactions between the particles with the vessel and the matrix melt. (C) 2014 Elsevier B.V. All rights reserved.